The strange filaments of the Milky Way have older, distant cousins

Large-scale magnetic filaments spill downward from a black hole's jet, located in a distant cluster galaxy. Credit: Rudnick and collaborators, 2022

Farhad Zadeh, an astronomer at Northwestern University, has been captivated and perplexed by a family of large-scale, highly structured magnetic filaments dangling in the center of the Milky Way since discovering them in the early 1980s. Zadeh is still curious, but perhaps a little less perplexed, 40 years later. With the finding of comparable filaments in other galaxies, Zadeh and his colleagues have given two plausible theories for the filaments' unknown origins for the first time. Zadeh and his co-authors argue that the filaments might be caused by an interaction between large-scale wind and clouds or by turbulence inside a weak magnetic field in new research published earlier this month in The Astrophysical Journal Letters.

We know a lot about the filaments in our own Galactic Center, and now filaments from other galaxies are showing up as a new population of extragalactic filaments, according to Zadeh. Despite their very diverse surroundings, the basic physical mechanics of both filament populations are comparable. Although the objects are related, the filaments outside the Milky Way are older, distant relatives, and I mean extremely distant (in time and space).

Zadeh is a physics and astronomy professor at Northwestern's Weinberg College of Arts and Sciences and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).

Zadeh identified the first filaments, which were up to 150 light-years long and towered around the Milky Way's center supermassive black hole. Zadeh added approximately 1,000 additional strands to his collection of observations earlier this year. The one-dimensional filaments appear in pairs and clusters in that batch, typically piled evenly spaced, side by side like harp strings or pouring sideways like individual ripples in a cascade. Zadeh found the mysterious filaments using radio telescope data. The filaments are made up of cosmic ray electrons gyrating along a magnetic field near to the speed of light. Despite the fact that he is piecing together the riddle of what the filaments are comprised of, Zadeh is still curious about where they originated from.

When scientists identified a new population beyond our own galaxy, it opened up new avenues for research into the physical processes occurring in the region around the filaments. The filaments were discovered within a galaxy cluster, a concentrated tangle of thousands of galaxies one billion light-years from Earth. Some of the galaxies in the cluster are active radio galaxies that appear to be breeding sites for large-scale magnetic filament development. When Zadeh first observed these freshly discovered filaments, he was astounded.

He stated, "I was quite pleased to witness these extraordinarily beautiful structures after researching filaments in our own Galactic Center for all these years." Because we discovered similar filaments everywhere in the cosmos, it suggests that something more general is going on.

Although the new filament population appears to be comparable to those in our Milky Way, there are several significant variations. Outside the Milky Way, for example, the filaments are 100 to 10,000 times longer. They are also considerably older, with lesser magnetic fields. The enormous emptiness of the intracluster medium, or the space wedged between the galaxies inside the cluster, is hung at a 90-degree angle from the jets of a black hole. However, the newly found population has the same length-to-width ratio as the filaments of the Milky Way. And it appears that both populations move energy using the same ways.

The electrons in the filaments are more energetic closer to the jet, but they lose energy as they move down the filament. Although the black hole's jet may provide the seed particles required to form a filament, something unknown must be speeding these particles over incredible distances.

According to Zadeh, some of them have incredible lengths of up to 200 kiloparsecs. That is around four or five times the size of our whole Milky Way galaxy. What's astonishing is how long their electrons can stay connected. It would take 700,000 years for an electron to travel at the speed of light down the length of the filament. They also do not travel at the speed of light.

Zadeh and his colleagues believe in the new research that the filaments' origins might be a simple interaction between galactic wind and an obstruction, such as a cloud. The wind generates a comet-like tail behind the obstruction as it coils around it.

Wind, according to Zadeh, is caused by the rotation of the galaxy itself. It's like sticking your hand out the window of a moving automobile. There is no wind outside, but the air is moving. When the galaxy moves, it generates wind, which may be pushed through areas where cosmic ray particles are relatively loose. It sweeps the material, resulting in a filamentary structure.

Simulations, on the other hand, provide another feasible option. Long, filamentary formations appeared when researchers recreated an active, turbulent medium. Gravity may alter and agitate the medium as radio galaxies move about, according to Zadeh. The material then generates whirling eddies in areas. After wrapping around these eddies, the weak magnetic field may be stretched, folded, and amplified, finally forming elongated filaments with powerful magnetic fields. Despite the fact that many questions remain, Zadeh is nonetheless awestruck by the latest discovery.

All of these filaments beyond our galaxy, he claims, are exceedingly ancient. They are virtually from a separate epoch of our universe, yet they show the Milky Way's inhabitants that the filaments have a similar beginning. This is extraordinary, in my opinion.

Journal Information: F. Yusef-Zadeh et al, Populations of Magnetized Filaments in the Intracluster Medium and the Galactic Center, The Astrophysical Journal Letters (2022). DOI: 10.3847/2041-8213/ac982a